The present invention relates to semiconductor memories. It combines the advantages of ferroelectric devices, such as non-volatility and radiation hardness, with the durability and other advantages of other types of memories such as static RAMS and dynamic RAMS.
Memory technology today includes several different techniques. The general population is aware that information can be stored on magnetic tape, as magnetic tape is used for video and audio recording. Such applications involve storing information in magnetic fields on a magnetic medium. In the case of audio or video applications, the stored information is written and read in a serial fashion so that consecutive video images or audio sounds can be recorded or played back. Magnetic tape is a non-volatile memory in that power need not be supplied continuously to this storage medium in order to preserve the information stored therein.
Similarly, bubble memories also store information in magnetic domains and are non-volatile. However, magnetic bubble memories, like magnetic tape, must be accessed serially.
It is a common practice today among computer users to store information or programs which have been developed or updated using their computers in a medium which is non-volatile so that in the event of a power outage or a mistake, data can be retrieved. This is one of the salient advantages of non-volatile memories. Magnetic floppy discs are commonly used for this purpose today.
However, high speed processing requires that information stored in memory be randomly accessible. Consequently, over the past several decades, memory technology has developed various types of random access memories. Generally these involve semiconductor technology, which operates at high speed, although magnetic random access memories may have been addressed. In semiconductor random access memories ("RAMS"), a bit of binary information is stored in a memory cell, and cells are grouped together into arrays. It is common practice today to include numerous bit lines, each bit line being coupled to several memory cells, and an orthogonal group of word lines, each of which is coupled to several memory cells. Various address decoders identify one cell to be accessed. Thus, by specifying an address, a RAM is able to access a single memory cell in an array of thousands of memory cells and read or write data from or into that addressed memory cell. This cell may be used repeatedly and accessed very quickly, sometimes in a few dozen nanoseconds. These capabilities are quite important to computer and data processing applications.
RAMS are further categorized as being either "dynamic" or "static." This distinction generally follows from the type of memory cell incorporated in the RAM. In the case of a dynamic RAM memory cell, data is stored in a capacitor, part of which is found in a substrate of semiconductor material. A transistor selectively couples the capacitor to a bit line. Because of this simple construction, dynamic RAM ("DRAM") memory cells are small in area and can be fabricated with substantial density. On the other hand, because the charge is stored in a capacitor in the substrate, the charge dissipates and needs to be refreshed periodically in order to preserve the content of the memory.
Static RAMS differ from dynamic RAMS by having memory cells which do not need to be refreshed. A static RAM cell usually includes several transistors configured as a flip-flop which has two stable states. These two states are used for storing the two different levels of binary data. Static RAM cells, because they include several transistors, are larger than DRAM cells and therefore cannot be packed as densely on semiconductor chips. On the other hand, static RAMS operate quickly and do not require the logic circuitry needed for refresh operations.
Both dynamic RAMS and static RAMS, while having the advantage of being randomly accessible, have the disadvantage of being volatile. That is, when power is removed from the memories, the data dissipates. The charge stored in the capacitors in the memory cells of the dynamic RAMS dissipates, and the voltage used to preserve the flip-flop states in the static RAM memory cells drops to zero so that, in short order, the flip-flop loses its data.
RAMS using ferroelectric capacitors for memory cells have a significant advantage of being non-volatile. Briefly, a ferroelectric capacitor includes a pair of capacitor plates with a ferroelectric material between them. A ferroelectric material has two different stable polarization states which can be defined with a hysteresis loop seen by plotting the polarization against applied voltage. By measuring the charge which flows when a voltage is applied to a ferroelectric capacitor, one can determine the polarization state of the ferroelectric material. By assigning a binary zero to one polarization state and a binary one to the other polarization state, ferroelectric capacitors can be used to store binary information. The advantage, of course, of a non-volatile memory is that even though power may be interrupted or removed from the memory, data will continue to be stored. Another advantage of ferroelectric materials in particular is that they have been found to be radiation hard.
However, a disadvantage is that certain ferroelectric materials have been found to exhibit fatigue characteristics which result in decreasing polarization as the ferroelectric capacitor is switched repeatedly from one polarization state to the other, millions of times.
It is an object of the present invention to provide a non-volatile semiconductor memory using ferroelectric materials, but overcoming the problem of polarization fatigue and having the advantages of random access and high speed.